(Not Applicable)
The present invention relates generally to air filtering systems, and more particularly to a sintered glass bead filter with active destruction capability for filtering microcontaminants from air to be breathed by humans.
Air filtering systems are important for healthy breathing in a number of environments and in a number of applications. For instance, large office buildings must incorporate air filtering systems designed to ensure that the air recycled within a building is clean, in order to protect the health of the people in the building. Hospitals must use air filtering systems to isolate weakened patients from pathogens, or isolate patients with contageous illnesses from other patients. Also, the rising incidence of terrorism in the United States has created awareness of the increasingly present possibility that civilian populations will be intentionally targeted with biological weapons in the near future. Of course, air filtering systems are particularly important to military personnel, who regularly operate in arenas where exposure to natural or artificial microcontaminants is a particularly real possibility. In each of these situations, the welfare or even the lives of the people involved depend entirely upon the quality of the air filtering equipment at their disposal. Accordingly, it is important that air filtering systems continue to advance to meet the new challenges posed as the world becomes a more complicated and dangerous place.
Prior art air filtering systems take variety of embodiments. The simplest kind of filter is a physical filter. The simple physical filter is composed of a fabric or other porous material. The pores of the material must be smaller than contaminants to be filtered, but large enough to allow air passage. If they are, the contaminants will be blocked, and only the clean portion of the air will pass the filter. Obvious disadvantages of the physical filter are the speed with which the filter becomes clogged, and therefore useless, and the fact that physical filters generally cannot screen particles of below a certain size.
A more advanced type of filter is the activated carbon filter. Activated carbon filters comprise a highly porous activated carbon element, the cavities of which effectively draw in contaminants by means of both London Force and electrostatic force in a kind of capillary action. Activated carbon filters are substantially more effective than simple physical filters at trapping many kinds of small contaminants. However, certain contaminants will still evade activated carbon filters.
Another more advanced type of filter is the ionic filter. Ionic filters produce negatively charged ions which attach to particles in the air. The air is then passed through a positively charged filter. The negatively charged ions are drawn toward the positive charge and carry the attached particles with them, removing them from the air as it passes through the positively charged filter. Like activated carbon filters, ionic filters are more effective against certain microcontaminants than simple physical filters, but still fail to neutralize others.
Still another advanced type of filter is the High Efficiency Particle Arrest (HEPA) filter. HEPA filters employ a glass fiber filter which is pleated in such a way that the actual surface area over which air passes is very large in comparison to the volume occupied by the filter. The large surface area results in a decreased pressure drop across the filter, or in other words, it is easier for air to pass through the filter. In its simplest form, the HEPA filter is essentially an advanced physical filter.
Modern mechanical filters generally employ multiple levels of filtering, often using more than one of the above mentioned methods in sequence. Complicated mechanical filters reach high levels of effectiveness in filtering air, but have the disadvantage of being too complicated for use in many applications.
In addition to any other drawbacks, all of the above mentioned filters suffer from the common drawback that they become clogged with contaminants over time, resulting in an air pressure drop across the filter. In other words, as the filter is used, it becomes more and more difficult to pass air through it. A recent technology for eliminating contaminants which addresses this problem involves using a transition metal oxide and water in conjunction with an ultraviolet light source in order to create free hydroxyl groups with microbicidal properties. This method is described in U.S. Pat. No. 5,933,702 PHOTOCATALYTIC AIR DISINFECTION issued to Goswami (xe2x80x9cGoswamixe2x80x9d). Ultraviolet light is cast upon a transition metal oxide in the presence of water. In response, the transition metal oxide undergoes a photocatalytic reaction with the water, thereby producing free hydroxyl radicals. The hydroxyl radicals react with contaminants, rendering them neutral.
The above described method has been put to use in a variety of applications. U.S. Pat. No. 6,235,351 METHOD FOR PRODUCING A SELF DECONTAMINATING SURFACE issued to DiMarzio et al. discloses surfaces which employ the method to become self decontaminating when exposed to ultraviolet light. Goswami discloses air filters made using the method. However, prior art photocatalytic disinfection systems have suffered from the drawback that because exposure to ultraviolet light is necessary for proper functionality, it was impossible to manufacture filters which combined the benefits of high surface area for minimum pressure drop and maximum simplicity in design with respect to lighting sources and manufacturing complexity. Accordingly, a need exists to devise a photocatalytic filtering system which can combine these features in order to make photocatalytic filtering more effective and bring it into broader applicability.
It is therefore an object of the present invention to provide a photocatalytic filtering system free of the aforementioned drawbacks. The filter system comprises a plurality of sintered glass beads having pores formed therebetween for the passage of air therethrough. In the preferred embodiment, the sintered glass beads are sintered at a temperature of above a transition temperature of the glass beads. Sintering at above the transition temperature ensures that a degree of crystalization within the glass beads is under a selected threshold. Crystal formation reduces transmissiveness, so the selected threshold is selected so that ultraviolet light can penetrate the glass beads to a selected depth in order to ensure that all the glass beads can be illuminated.
The sintered glass beads essentially form a highly microporous glass structure. The surface area of the glass beads per thickness of the structure is high compared to what it would be if there were no pores formed therebetween. Because pressure drop is inversely proportional to the surface area of the glass beads, the pressure drop characteristic of the filter system benefits thereby. Air follows a tortuous path between the glass beads while being filtered as described below. Depending on the size of the pores, the sintered glass beads may also act as a physical filter.
The filter system further comprises a coating formed on the glass beads of transition metal oxide, such as TiO2, and water. The water can be provided by ambient humidity or artificial humidification. An ultraviolet light source is also comprised by the system, and may be the sun or an artificial source. It is used to cast ultraviolet light upon the glass beads. Because the glass beads are substantially transmissive of ultraviolet light, at least some ultraviolet light will pass the more proximate glass beads to reach the more distal glass beads. Accordingly, the surface area of all the glass beads is illuminated. The ultraviolet light causes a photocatalysis reaction between the titanium oxide and the water of the coating, producing free hydroxyl radicals with microbicidal properties. The hydroxyl radicals accomplish the active destruction of microcontaminants.
The filter of the filter system may be configured advantageously through methods known to those in the art. For instance, the filter may be designed as a hollow cylinder. The hollow cylinder design has a high surface area compared to that of other possible shapes, reducing pressure drop. If the pressure drop of the filter is below 40 mm H2O at 85 liters per minute, unassisted human breathing through the filter is possible. A filter of this shape with 2 cm walls and a 15 cm height proved to have a pressure drop well within this limit while retaining above 98% capture efficiency at all face velocities.
In accordance with a further embodiment, urethane foam is inserted between the glass beads prior to sintering. During sintering, the urethane foam decomposes and oxidizes. The result of using urethane foam is a bimodal pore size distribution. The paths between the glass beads appear to take more tortuous paths when there are both large and small pores, increasing capture efficiency
In still another embodiment, particulates disposed on the glass beads are also comprised by the filter system. The particulates can be, for instance, glass particulates or chopped fibers. The particulates alter the surface activity of the filter system, and may increase capture efficiency in certain applications.